This application is a Section 371 of PCT/NL 99/00696 filed Nov. 12, 1999.
The invention relates in the first place to a method for removing floating substances and salts from a liquid by means of a membrane filtration.
It is known, for removing salts from a liquid by means of nano filtration and reverse osmosis or hyperfiltration, to use spiral wound and hollow-fiber modules. Since, when using such spiral wound and/or hollow-fiber modules, the liquid has to be free from floating substances (since these modules could become clogged) a drastic precleaning of the liquid is necessary, for example by means of a sand filtration and/or micro-filtration or ultra filtration, respectively. This means that a two step method would be necessary.
One of the problems which are encountered with membrane filtration is the so-called concentration polarization of the components present (like salts, floating substances and other substances). Because of the liquid flow through a membrane (permeate production) salts are transported in the direction of the membrane. This brings about a concentration which is near the membrane higher than the average concentration in the bulk. This occurrence is called concentration polarization. When the solubility, product of the respective salt is exceeded, this salt tray precipitate onto the membrane and have a negative influence on the functioning of the membrane. The precipitation of the salt, can be prevented by adding chemicals (which comprises a reduction of the supersaturation in the bulk by adding an acid and/or an anti-precipitant) or by means of a so-called cross-flow operation.
The known two step method has several disadvantages. Both during the first step (sand filtration and/or microfiltration or ultra filtration, resp.) as well as during the second step (nano-filtration or hyper-filtration, respectively) a production of concentrate takes place. The total concentrate production which is finally obtained is therefore high. Furthermore, production with spiral wound and hollow fiber modules always takes place under continuous conditions (i.e, a continuous production of permeate and a continuous removal of concentrate) so as to decrease the concentration polarization as mentioned herein before. Such continuous operation conditions lead to a high energy consumption and, because of the pressure decrease (comprising a concentrate control valve) at the concentrate removal side, a significant energy loss is obtained. Finally, it is shown in practice that the spiral wound and/or hollow fiber-modules must be exchanged quite often, which renders installations which comprise such modules rather expensive in use.
The invention aims to a method with which the disadvantages as mentioned herein above can be solved in a simple, but effective way.
To this end, the method according to the invention is characterized in that use is made only of vertically positioned, spiral wound and plate and frame type membranes which are suitable for nano- or hyper filtration, preferably capillary oz tube membranes, wherein repeatedly and consecutively the steps are performed which comprise:
a. producing a permeate without concentrate discharge during a predetermined period of time;
b. then guiding a gas, for example air, alongside the membrane, at least almost without permeate discharge and without concentrate discharge;
c. finally discharging concentrate without discharging permeate.